Unless I am overlooking some simple tool or process, it is *way* harder than it should be in the year of our lord 2022 to take composite two video files and make a single split-screen clip with the audio synced at a specific time.
Probably easy in pro video editing software, but I don't have that and various free options are (as far as I can tell) absolute garbage for fine-tuning composite effects, especially if you're trying to sync based on an audio waveform.
The super vexing thing is that I can't even get this working with ffmpeg despite knowing where I want to start each video down to the millisecond. (I think it has to do with keyframes. The audio syncs up fine, but the video winds up out of sync with the audio.)
Anyway, I spent an hour or so recording one of Herbert J. Ellis' 
nice little banjo duets yesterday evening and I've spent like 5 hours so far trying to just assemble them into a single video clip
... I feel just plain incompetent at this point.
Wicked frustrating. This was supposed to be a quick "post online for the dopamine hit" effort but I don't feel much closer to getting both video tracks in sync with the combined audio than I did last night.
I'm at the "really need to delete all of the files I've generated and start over with the original clips" stage. It's *got* to be some issue at the intersection of key frames and frame rates, combined with my personal lack of knowledge about how video/audio containers work.
But, now I need to go be social and focus on things that are not this for the next day and a half. 😒
I may try to at least post an audio mix later.
Meh, sync is off by a couple of frames (probably because I split the audio out to tweak it and then brought it back in?) and this is not a proper audio mix, but I'll try again later.
In honor of the fireflies being out this week, here is "Firefly Jig" by Herbert J. Ellis , written circa 1900.
@mayor The "pro" way to do it would be to have a sync click at the start of each video. Use the clicks to align the videos.
@Unatributed I did this by doing an audible count-in for each video - I got the two parts in sync (apart from a bit of tempo drift here and there in my own playing) early on, the issues have all been with the video getting out of sync with the audio during processing.
Nothing to post yet (going to try to actually mix the quality audio after all this trouble and) but I sorted out the sync problem:
trick is to re-encode the original video with the ffv1 codec which is lossless (I think? It makes for huge files anyway) and allows for much more precise seeking. Then mash them together, and re-compress the output